A multitude of devices exist that continuously transmit data that is repeated over and over. These devices are typically used for telemetry applications using RFID as the transmission medium. In decoding this continuous data stream, a common problem is how to identify the beginning of a continuously repeating data stream.
For example, access control systems typically implement an RFID transponder embedded in a card referred to as an access control card. A card reader is located near an access point in a premises, such as by a door in a building. When the holder of the card presents the card towards the reader, an RF field generated by the reader is used by the transponder in the card to generate response data that is encoded with a unique card identifier. The card ID is used to modulate a return RF signal that is received, demodulated and decoded by the card reader circuitry and then used to lookup a database (centrally located), for example, and determine if that user is authorized to enter the door under control. If authorized by the system, the access control reader will allow opening of the door (such as by electronically unlocking the door) so the user can enter. If that user is unauthorized, then the door will not be opened for the user. These proximity access control systems are well known in the art and need not be described in further detail herein.
The nature of this type of system is that the response data generated by the transponder in the access control card is continuously sent, since it is a function of the RF field generated by the card reader device. As such, when the transponder enters the area it will continuously send the response data, and the card reader will not be able to determine the beginning and end of a data sequence. This problem has been addressed in the past by using known preamble sequences in the card, so that the card reader will look for the predetermined sequence in order to determine the proper data that follows the preamble. Other mechanisms such as framing bits or sequences may also be used in the prior art to determine appropriate synchronization.
It is desired, however, to be able to determine the beginning of a data stream in this type of system that does not rely on the use of unique data patterns such as a preamble and the like.